What Exactly is Prepreg in a PCB?

Before we can understand a prepreg, we need to know what a PCB is. A Printed Circuit Board (PCB) is what makes all of the electronics you use today operate. It is a series of electrical components interconnected onto a small device. Without the PCB, our electronics wouldn’t be able to function as intended.

A PCB is mostly made up of multiple layers of the substrate, copper layering, small holes drilled into the device, solder mask, a coating (quite often in tin-lead), multiple components, before being fused together. Although, when put this way, developing a PCB doesn’t seem like it involves too much. However, the actual creation process is extensive, as you must be precise with every calculation to avoid malfunction.

What is a Prepreg PCB?

A prepreg (pre-impregnated) is one of the main materials used in multilayer boards and is what holds the cores together. It is composed of fiberglass impregnated with resin (an epoxy-based material). The layers get pressed together at a temperature to create the required board thickness.

The prepreg and core quite often get mixed up. However, they are two separate components of the PCB. The core of the PCB is the FR4 layers of copper traces and glass-reinforced epoxy laminate sheets. Once heated, the prepreg holds the core of the PCB and the layers together.

Application in the PCB Manufacturing Process

Multi-Layered Boards

The prepreg in a multi-layered board is what holds the layers together. Once you stack all the layers of the board, you fuse them together by exposing the board to high temperatures. The thickness of the prepreg will be in accordance with the thickness of the overall board.

The prepreg should have specific characteristics to it when used on a multi-layered board. The cloth surface should look and feel smooth and be free of any oil, stains, defects, foreign matter, cracks, or excessive resin powder.

Complex PCBs

If you have a complex PCB, you’ll likely have a more complex prepreg. You’ll need to use different types of the prepreg to achieve the required thickness of both the prepreg and the board itself.

How it is Used

Prior to use, the prepreg will be dry but not hard. This allows the prepreg to flow through the PCB when heated. The fiberglass gets impregnated with the resin before being put onto the layer. Once aligned, the prepreg is heated and put under pressure to soften the material. Once done, as the PCB cools, the prepreg begins to solidify and hold everything together.

Why is it Important?

The prepreg plays an essential role in the development process by the PCB manufacturer. Without the prepreg, there would be nothing there to hold the layers together.

What Role Does it Play?

The role of the prepreg is to fuse etched cores together or to glue a copper foil to an etched core. You will find a layer of prepreg between each layer of the PCB.

Conclusion

The prepreg is not just an important part in the PCB manufacturing process, it is a necessity if the board has multiple layers. Without the prepreg, there would not be anything holding the different layers together.

The prepreg and core are two different parts of the PCB. For starters, the core is an FR4 material with copper traces. Whereas, the prepreg is fiberglass impregnated with resin. It is the prepreg that holds the core together on the PCB.